Conventionally, intermittent slow reproduction has been performed in a helical scanning type VTR. The intermittent slow reproduction is achieved by driving intermittently a tape so that still reproduction and normal reproduction are repeated with a predetermined interval. In order to perform such intermittent slow reproduction, the VTR is generally provided with an intermittent driving circuit. Briefly stated, the intermittent driving circuit functions such that a tape is initiated in response to a normal head switching signal (RF switching pulse) to perform normal reproduction and the tape is stopped in response to a control signal reproduced at the time of tape traveling to perform still reproduction.
More specifically, when normal reproduction is performed, the intermittent driving circuit generates a start pulse in response to the head switching signal and applies the same to a capstan motor of direct drive so that the capstan motor is rapidly rotated. On the other hand, when still reproduction is performed, the intermittent drive circuit generates a brake pulse in response to the reproduced control signal and applies the same to the capstan motor so that the capstan motor is reversed to brake the tape. During a tape traveling period between the above described start pulse and the brake pulse, the speed of the capstan motor is controlled utilizing an FG (frequency generator) signal of the capstan motor as described below.
In the above described intermittent slow reproduction, the position where the tape is stopped in a still reproduction state is important. More specifically, in the still reproduction state, unless noise is forced to be included surely in a vertical branking period, noiseless intermittent slow reproduction can not be achieved. The position where the tape is stopped in the still reproduction state is largely affected by the pulse width of the brake pulse. More specifically, if the pulse width of the brake pulse is smaller than the proper value, the tape is stopped prior to the correct position, and if it is larger than the proper value, the tape traveling direction is reversed.
The conventional VTR has been adapted such that a two-phase FG signal is extracted from the capstan motor, reverse of the capstan motor is determined by the two-phase FG signal, and application of the brake pulse is stopped when reverse is detected. Such a conventional technique is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 83367/1983; "Sharp Technical Journal" , of Japanese Periodical, No. 29, 1984, pp. 125-129 ; "National Technical Report" , Vol. 28, No. 3, Jun. 1982; and Japanese Patent Laying-Open Gazette No. 76984/1982.
FIG. 1 is a schematic block diagram showing a circuit for detecting reverse of the capstan motor used in the above described conventional intermittent driving circuit. Description is now made on the structure of the circuit shown in FIG. 1.
In FIG. 1, the periphery of a magnet 1 which rotates simultaneously with a capstan motor (not shown) is magnetized to a number of poles. In addition, magnetic reluctance elements 2 arranged with a predetermined spacing from each other are provided, whose outputs are supplied as FG signals out of phase with each other by 90.degree. through amplifiers 3 and 4 and comparators 5 and 6 for waveform shaping. Of the FG signals, a signal FG1 is applied to a data input of a D type flip-flop 7 and a signal FG2 is applied to a clock input of the D type flip-flop 7. A Q output of the D type flip-flop 7 is supplied as a determination output of reverse of the capstan motor.
FIG. 2 is a waveform diagram for explaining operation of the circuit shown in FIG. 1.
Referring now to FIG. 2, description is made on operation of the circuit for detecting reverse of the capstan motor shown in FIG. 1. The D type flip-flop 7 latches a level of the signal FG1 applied to the data input at the rising edge of the signal FG2 applied to the clock input. Therefore, as seen from FIG. 2, when the capstan motor rotates in a forward direction, an H level of the signal FG1 is necessarily latched by the D type flip-flop 7 at the rising edge of the signal FG2 and is outputted as an output of determination of forward rotation. However, when the capstan motor is reversed by applying the brake pulse and an L level of the signal FG1 is latched at the rising edge of the signal FG2, the D type flip-flop 7 applies an output for informing reverse detection of the capstan motor. The intermittent driving circuit stops application of the brake pulse to the capstan motor in response to reverse detection of the capstan motor.
However, since in the above described conventional intermittent driving circuit, the level of the other FG signal (for example, the signal FG1) can be latched, that is, the rotational direction of the capstan motor can be detected only at the timing of one edge of one of two FG signals (for example, the rising edge of the singal FG2 in FIG. 2), the time required for obtaining the reverse detecting output after the capstan motor is actually reversed may be long, in which case the brake pulse with a proper pulse width is not liable to be obtained. Furthermore, in the conventional intermittent driving circuit, when the FG signal at the time of starting the capstan motor is unstable and noise is contained, the determination circuit shown in FIG. 1 erroneously operates, so that there may be a possibility of failure of tape initiation.